Smooth muscle cell (SMC) proliferation is a key event in neointimal formation following balloon angioplasty. The molecular signals that mediate this process have yet to be identified. Mitogen-activated protein (MAP) kinases, a group of tyrosine/threonine kinases, are rapidly activated in cultured cells in response to mitogens and stress. These kinases are thought to play a pivotal role in transmitting transmembrane signals required for cell proliferation. The present studies were designed to investigate whether MAP kinase is involved in the development of the neointima in injured arteries. Rat carotid arteries were isolated at variou times following balloon injury, and MAP kinase p42 (ERK2) activity in protein extracts of the vasculature was determined using a protein kinase assay and Western blot analysis. Following balloon angioplasty, MAP kinase activity in the vessel wall increased rapidly, reached a high level in 5 min and were maintained for 1 hour. A sustained increase in MAP kinase activity was also observed over the next 7 days and returned to baseline by 14 days after injury. In contrast, contralateral and uninjured arteries did not show significant changes in MAP kinase activity. Concomitantly, Western blot analysis revealed a slower migrating 42 kDa protein confirming that MAP kinase in the injured vessels was indeed activated or phosphorylated. This kinase activation may be crucial in initiating SMC proliferation in response to angioplasty.